Line of Duty fans were devastated when series four's finale hinted that stalwart copper Ted Hastings might actually be corrupt.

It seems that Adrian Dunbar's character in the BBC One thriller – an old-fashioned copper with a set of memorable catchphrases – has developed a following all his own.

"It's come as a complete surprise, a lovely surprise!" the actor told Digital Spy. "I don't quite know how to respond to it, other than I'm delighted that people like the character, 'cause I have great fun playing him.

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"He's pretty old-school. Not ever intentionally meaning to offend, of course, but the police force in particular has to be sensitive to all shades of colour and opinion and so forth. So he's a bit of an anachronism. But I think people kind of like that, because they can see that he's not holier than thou, by any means."

"I don't think that I picked up on Jed's initial thinking on the character through the script," Dunbar said. "He may've seen him one way, but it was possible to see a number of ways to play the character, I'm sure.

"I thought that he was kind of uncompromising on the one hand, but on the other hand, terribly loyal to his people. [Those are] the kind of qualities needed to run something like AC-12, I think. If he's the head of a unit like that, where nobody likes them and they're isolated, you have to be pretty tough in some respects.

"Your life can be falling apart around you and yet you have to have the bottle to see things through at work, and see them through to the end."

Dunbar's portrayal of Hastings has proven to be one of Line of Duty's biggest assets, with fans loving his catchphrases ("Hastings – like the battle!") and colloquialisms – in particular, his habit of referring to people as "fella".

"Now and again, we do things just off the cuff, bit of spontaneity here and there, and then things gradually become part of the character," Dunbar explained. "'Fella' does seem to have caught on – an American on Hampstead Heath this morning said, 'Hiya, fella!' to me, and I thought, oh God, 'fella' definitely has caught on!

"It's fun to bring a bit of that colloquial quality to it, that regional stuff... it adds character to everything that we're doing. So I really enjoy all of that."

He's rarely tempted, though, to slip an extra "fella" or two into Mercurio's scripts. "They're usually scripted. I have to keep an eye on all those things, so you're not doing it all the time. It would become very wearing and boring, I think, if we were at it all the time. So you have to ration your fellas!"

"We've been doing all these little scenes, three or four pages, and you know that that 30-pager's coming up, and you're going to be sitting in a glass box with hot lights on it for two days and you're going to dehydrate. You end like up like a prune!

"They really are big set pieces and they are pretty exhausting, but it's the signature of the show, those interrogation scenes, and long may they continue."

As for the second point, Dunbar admitted that all of the Line of Duty cast – even the central trio of himself, Vicky McClure (DS Kate Fleming) and Martin Compston (DS Steve Arnott) – fear that their characters could be killed off at a moment's notice.

"Yes, of course, we all do. Every series, the first two or three scripts come in and you're just reading it, thinking, 'It looks like I'm going to survive – that's good!' – and you don't see all the scripts from the get-go.

"It takes a while, we're drip-fed the scripts as they come in, which makes it very immediate in the playing. Somebody we're talking to now could actually be betraying us in four episodes' time!"

Line of Duty series 4 and complete series 1-4 box set are available now on DVD.